Display device and controlling method thereof

ABSTRACT

A conventional setting voltage was a value with an estimated margin of a characteristic change of a light emitting element. Therefore, a voltage between the source and drain of a driver transistor V ds  had to be set high (V ds ≧V gs −V Th +a). This caused high heat generation and power consumption because a voltage applied to the light emitting element. The invention is characterized by feedbacking a change in a current value in accordance with the deterioration of a light emitting element and a power source voltage controller which modifies a setting voltage. Namely, according to the invention, the setting voltage is to be set in the vicinity of the boundary (critical part) between a saturation region and a linear region, and a voltage margin for the deterioration is not required particularly for an initial setting voltage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.12/255,235, filed Oct. 21, 2008, now allowed, which is a continuation ofU.S. application Ser. No. 11/675,116, filed Feb. 15, 2007, now U.S. Pat.No. 7,453,453, which is a continuation of U.S. application Ser. No.10/697,003, filed Oct. 31, 2003, now U.S. Pat. No. 7,180,515, whichclaims the benefit of a foreign priority application filed in Japan asSerial No. 2002-318974 on Oct. 31, 2002, all of which are incorporatedby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display device (light emittingdevice) having a light emitting element, and more particularly such adisplay device having a display part of current driving method.

2. Description of the Related Art

Digital gradation method (digital driving method) and analog gradationmethod (analog driving method) can be given as driving method ofmulti-color expression on a display device having a light emittingelement. In the aforementioned digital gradation method, a lightemitting element is driven in binary of ON (brightness is almost 100%)and OFF (brightness is almost 0%) to obtain gradation by controlling theluminous region and the length of the period during which each pixelemits light. In the analog gradation method, analog input data iswritten into a light emitting element to modulate the gradation in ananalog manner.

Furthermore, expression of gradation is made through two methods, whichare a constant voltage drive which is dependent on the voltage appliedto the light emitting element, and a constant current drive which isdependent on the current applied to the light emitting element. Anelectric current flowing through a light emitting element is controlledby a transistor (hereinafter referred to as driver transistor) incurrent drive.

Operation of a driver transistor is explained referring to the V-Ifeature shown in FIG. 8. There are two operating regions of the drivertransistor, namely, a saturation region and a linear region.

Linear region is a region of which current value changes according tothe voltage between the source and drain (V_(ds)) and the voltagebetween the gate and source (V_(gs))·(|V_(ds)|<|V_(gs)−V_(Th)|) In thelinear region, the following expression (1) is established. Note thatI_(ds) is the amount of current running through a channel formingregion. Note also that β=μC_(o)·W/L is established and μ thereof is amobility of the driver transistor, C_(o) is a gate capacity per unitvolume, and W/L is a ratio of the channel width W to the length L inchannel forming region.I _(ds)=β{(V _(gs) −V _(Th))V _(ds)−½·V _(ds) ²}  (1)

According to the expression (1) above, V_(ds) and V_(gs) obtain thecurrent value in the linear region. In the linear region, the lesserV_(ds) becomes, the lesser current value becomes too, while the currentvalue hardly increases even if V_(gs) gets larger.

When the driver transistor is operated in mainly the linear region, theamount of current flowing between both electrodes of the light emittingelement is changed according to both values of V_(gs) and V_(ds). Thedriver transistor is used as a switch, and a power source line and thelight emitting element are shorted if necessary, thereby flowing acurrent into the light emitting element. The current value flowingthrough the light emitting element is directly influenced by thecharacteristics (variation and deterioration in the manufacturingprocess) of the light emitting element that is connected to the drivertransistor.

In the saturation region, the current value is not changed by thevoltage between the source and drain (V_(ds)), in other words, it isonly changed by the voltage between the gate and source(V_(gs))·(|V_(ds)|>|V_(gs)−V_(Th)|)

In the saturation region, the following expression (2) is established.I _(ds)=β(V _(gs) −V _(Th))²  (2)

As set forth in the expression (2), the current value in the saturationregion is greatly dependent on a change in V_(gs) but not dependent on achange in V_(ds). Therefore, the current value in the saturation regionis not influenced by the characteristics of the light emitting elementconnected to the driver transistor.

On the other hand, when the driver transistor is operated in mainly thesaturation region, the amount of current flowing between both electrodesof the light emitting element is greatly dependent on a change in V_(gs)of the driver transistor but not dependent on a change in V_(ds). A gatevoltage of the driver transistor is controlled to flow the necessaryamount of current into the light emitting element. In other words, thedriver transistor is used as a voltage control current source and thedriver transistor is set such that a constant current flows between apower source line and the light emitting element.

In the constant current drive utilizing the abovementioned feature, thecurrent value is not dependent on a change in V_(ds) when the drivertransistor is operated in the saturation region. Therefore, the amountof current flowing into the light emitting element can be constantregardless of the characteristics (variation in the manufacturingprocess, deterioration, and temperature variation) of the light emittingelement.

When V_(gs) of a driver transistor is changed appropriately, the drivertransistor can be operated in mainly a linear region or in mainly asaturation region.

Operating a driver transistor in the saturation region as shown above isdisclosed in patent document 1.

[Patent Document]

Japanese Patent Laid-Open No. Hei 14-108285

In the abovementioned constant current drive, an operating region of atransistor steps into the linear region once V_(ds) thereof is decreasedto a certain point by the deterioration of an light emitting element. Toavoid this, a setting voltage of V_(ds) (V_(ds) of a driver transistorin operation) is set with an estimated deterioration (voltage fordeterioration, voltage α) of the light emitting element. The voltage αis dependent on the deterioration of the light emitting element.

In a conventional setting voltage, in short, V_(ds) needed to be sethigh because of the estimated value (812) for the margin of the changein characteristics of a light emitting element between before (810) andafter (811) deterioration. (|V_(ds)|≧|V_(gs)−V_(Th)+α|)

The voltage applied to the cathode and anode of a light emitting elementthus became inevitably high, causing heat generation and high powerconsumption.

It is an object of the invention to provide a pixel structure which canbe operated without adding the voltage α to the setting voltage for thedeterioration of the light emitting element. Namely, a pixel structurewith the setting voltage in the vicinity of the boundary between thesaturation region and the linear region (813 in FIG. 8) is to beprovided. A further object of the invention is to provide a displaydevice provided with an aforementioned pixel and a control methodthereof.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above problems, andhas an object thereof to modify a setting voltage by providing a powersource voltage controller which feedbacks the change in current value inaccordance with the deterioration of the light emitting element and setsthe setting voltage thereby. Therefore, the setting voltage in thevicinity of the boundary between the saturation region and the linearregion is to be provided, without the margin of voltage α for thedeterioration particularly in the initial setting voltage.

To put it concretely, the invention utilizes an element to check thedeterioration of a light emitting element (hereinafter referred to as amonitoring element) and controls the power source voltage in accordancewith the deterioration of the monitoring element. That is, voltagebetween the source and drain is modified to a constant value by fixingthe potentials of a gate electrode and source electrode of the drivertransistor of the monitoring element and controlling the potential of adrain electrode (drain terminal) in accordance with the deterioration ofthe light emitting element.

FIG. 1 is a pattern diagram of the structure of the invention, showing apixel portion 103 having a monitoring element 101 and a pixel 102. Themonitoring element 101 has a light emitting element and a drivertransistor connected to the pixel. The pixel 102 also has a lightemitting element and a driver transistor connected to the pixel. Theinvention has a first electrode 104 and a second electrode 105 connectedto the monitoring element 101 and the pixel 102. A potential of thefirst electrode is shown as V₁, and a potential of the second electrodeis shown as V₂. Note that a monitoring element may be set up at any partincluding outside of the pixel portion.

Furthermore, the invention has a power source voltage controller 106 soas to keep the current value constant by recognizing the change incurrent value in accordance with the deterioration of a monitoringelement. Namely, the change in current value with the deterioration ofthe monitoring element 101 is fed back to the power source voltage ofthe pixel, fixing the potential of the first electrode: V₁, and changingthe potential of the second electrode: V₂. As the second electrode 105is connected to the monitoring element 101 and the pixel 102, currentvalue of the pixel 102 is kept constant by changing V₂.

Concerning FIG. 1, a layout of the pixel and monitoring element and thestructure of the elements are to be identical, while connections (withor without connections) may vary. Concerning the invention, however, thestructures of the pixel and the monitoring element do not necessarilyhave to be identical. However, in the case of forming the monitoringelement with the identical structures and different connections,manufacture thereof can be easier as there is no need to change theprocess but only the design of contacts and the like need to be changed.

Operation to control the power source voltage is now explained withreference to the flow chart, FIG. 2.

First, voltage to apply to the light emitting elements of the monitoringelement and a pixel is set (driving voltage of light emitting elements).At this time, the driver transistor is set to operate in the saturationregion, but the deterioration margin (voltage α) is not necessarilyneeded. That is, the voltage α which was conventionally necessary can beunnecessary or reduced according to the invention.

After that, a signal is inputted to the monitoring element and the lightemitting element of the pixel to emit light. The gradation expressionmethod to express multicolor at pixels may be either time gradationmethod or analog gradation method.

The light emitting element of the pixel as well as of the monitoringelement deteriorate gradually as time passes. As the light emittingelements of the pixels at this time are expressing gradations, few ofthem emit light constantly. On the other hand, a light emitting elementsof the monitoring elements are controlled to emit light at all time.That is, the light emitting element of the monitoring elementdeteriorates the fastest. Taking that into account, the power sourcevoltage is controlled to set the setting voltage in accordance with thedeterioration of the light emitting element of the monitoring element.In this way, the setting voltage can be modified in consideration of thedeterioration of the light emitting element of the pixels.

Deterioration of the light emitting element of the monitoring elementraises the resistance value of the light emitting element, lowers I_(ds)of the driver transistor, and reduces V_(ds) of the driver transistor.At this time, the setting voltage is to be adjusted by a power sourcevoltage controller to bring the current value to the setting current.That is, V₂ is to be reduced and the voltage applied to the lightemitting element is to be raised. Furthermore, the monitoring elementand the pixel have the same V₂, so that the setting voltage of the pixelis modified simultaneously.

It is to be noted that in the invention, the power source voltage may becontrolled by recognizing the change in voltage value or thecharacteristics in accordance with the deterioration of the monitoringelement. The power source voltage may also be controlled by the otherchanges besides the changes in voltage value and current value inaccordance with the deterioration of the monitoring element.

As described above, the invention enables the driver transistor tooperate in the saturation region without adding the deterioration margin(voltage α) to the setting voltage when the light emitting elementstarts emitting light. Therefore, the margin of the setting voltage dueto the deterioration of the light emitting element is not neededanymore. In general, the voltage α for the deterioration margin isestimated at 2 to 6V, which causes the driving voltage to decrease asmuch. As a result, heat generation and high power consumption at pixelscan be avoided. As heat generation of the driver transistor can bereduced particularly, the deterioration of the light emitting elementcan be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pattern diagram of a pixel portion of the invention.

FIG. 2 is a flow chart showing an operation of a display device of theinvention.

FIGS. 3A and 3B are pattern diagrams of a pixel portion of a displaydevice of the invention.

FIG. 4 is a pattern diagram of a pixel portion of a display device ofthe invention.

FIG. 5 is an equivalent circuit diagram of a pixel portion of a displaydevice of the invention.

FIG. 6 is a top plan view of a pixel portion of a display device of theinvention.

FIGS. 7A and 7 B are top plan views of a display module of theinvention.

FIG. 8 is a view showing a V-I feature of a transistor.

FIGS. 9A to 9H are views showing electronic apparatuses having pixelportions of display devices of the invention.

FIG. 10 is a pattern diagram of a pixel portion of a display device ofthe invention.

FIGS. 11A and 11B are pattern diagrams of a pixel portion of a displaydevice of the invention.

FIG. 12 is a view showing an experimental circuit of the invention.

FIG. 13 is a chart showing a change of an anode potential (V_(cathode))with the passage of time (hour).

FIG. 14 is a chart showing a current value (I) which is supplied to alight emitting element with the passage of time (hour).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the invention will be hereinafter describedreferring to the accompanying drawings. Note that an active element hasa gate, a source and a drain, but it is impossible to distinguish asource electrode and a drain electrode because of the elementstructures. Therefore, when a connection between the terminals isexplained, either of the source electrode or the drain electrode isreferred to as a first electrode, and the other is referred to as asecond electrode for convenience.

Embodiment Mode 1

Hereinafter explained with reference to FIG. 3 is an example in which anoperational amplifier is used as a power source voltage controller. Notethat in this embodiment mode, a p-channel type driver transistor isapplied, but an n-channel type driver transistor can be applied instead.Meanwhile, as an example of the invention, an anode potential of a pixel311 (V_(anode)) is referred to as V₁, and cathode potentials of thepixel 311 and of a monitoring element 301 (V_(cathode)) are referred toas V₂.

FIG. 3A shows an equivalent circuit diagram of a pixel structure of theinvention. A monitoring element 301 has a driver transistor 302 and alight emitting element 303 which is connected to a second electrode ofthe driver transistor 302. A pixel 311 has a driver transistor 312 and alight emitting element 313 which is connected to a second electrode ofthe driver transistor 312. The light emitting elements 303 and 313 areconnected to an output terminal of the operational amplifier 320 andhave a voltage of V₂. Meanwhile, first electrodes of the drivertransistors 302 and 312 have potential V₁ which is the same potential asthe electrodes of the light emitting elements.

A non-inverted input terminal (+side) of the operational amplifier 320is connected to the first electrode of the driver transistor 312 of thepixel, and an inverted input terminal (−side) is connected to the firstelectrode of the driver transistor 302 of the monitoring element. Thedriver transistor of the monitoring element is connected to a referencepower source (V_(ref)) via a resistance (R). Note that V_(ref) is higherthan V₁ (V_(anode)).

Next, a method for setting a voltage of the driver transistor of themonitoring element (V_(ds)) is explained.

First, V₁ (namely, V_(anode) of the monitoring element) is set on thebasis of a specification of a display device. Specifications of generaldisplay devices prescribe V₁ as 2 to 6V. A gate voltage of the drivertransistor 302 (V_(moni)) is set so that a predetermined current(I_(ref)) can flow in the monitoring element in a saturation region.Then, a gate voltage of the driver transistor 312 (V_(pix)) is set atthe same value or higher than V_(moni).

Next, the reference power source V_(ref) and the resistance value R areset so that the predetermined current (I_(ref)) can flow in themonitoring element 301 and the driver transistor 302 can operate in asaturation region. Note that other means to supply a predeterminedcurrent which flows in the monitoring element can be used besides thereference power source V_(ref) and the resistance R. For example, thepredetermined current (I_(ref)) can be supplied by a current source 321as shown in FIG. 3B.

Namely, the values of V₁, V_(ref), V_(moni), V_(pix), and R aredetermined by a practitioner and I_(ds) of the driver transistor 302 ofthe monitoring element is controlled externally.

As described above, a display operation is started by setting themonitoring element (called an operation state or a driving state). Afterthat, the light emitting element 303 of the monitoring elementdeteriorates as time passes. Similarly, the light emitting element 313of the pixel deteriorates. Further, due to the deterioration of thelight emitting element, the resistance value becomes high, thus acurrent value flowing in the monitoring element becomes low.

The operational amplifier will be hereinafter explained. The operationalamplifier 320 basically functions with an input potential ofapproximately 0 and with a non-inverted input terminal and an invertedinput terminal, the voltages of which are approximately the same.Accordingly, the following expression is established.I _(ref)=(V _(ref) −V ₁)/R=(V ₁ −V ₂)/R _(moni)∴V ₂=(R _(moni) /R+1)·V ₁−(R _(moni) /R)·V _(ref)

R_(moni) is a resistance value between the first electrode of the drivertransistor 302 and the power source side of the light emitting element303. The operational amplifier changes the value of V₂ when R_(moni) ischanged and keeps the I_(ref) value constant.

Such an operational amplifier modifies a setting voltage and furthermodifies V_(ds) of the driver transistor of the pixel because themonitoring element and the pixel has V₂ (V_(cathode)) in common.

Meanwhile, in this embodiment mode, V_(ds) is set in a saturation regioneven when electric characteristics of the driver transistor are changedby the temperature or the like because I_(ds) is determined andcontrolled by the operational amplifier.

Hereinafter, the setting current of the monitoring element will becomplimented in view of the difference of the deteriorating speedsbetween the pixel and the light emitting element.

In the case of a digital gradation method for example, the lightemitting element of the pixel repeatedly performs the light emission andnon-light emission (erasure) on the basis of a signal current (videosignal). Meanwhile, the light emitting element of the monitoring elementconstantly emits light. Therefore, the light emitting element of themonitoring element deteriorates faster than that of the pixel. Thatmeans the deterioration of the light emitting element of the monitoringelement is the fastest of all.

Namely, a voltage with an estimated deterioration of the light emittingelement can be set in the driver transistor of every pixel when V_(ds)of the driver transistor is set by the operational amplifier so that thedeterioration of the light emitting element of the most deterioratedmonitoring element can be offset. Therefore, it is not required tocontrol the light emitting element of the monitoring elementcorresponding to the light emission and non-light emission of the lightemitting element of the pixel.

However, preferably required is the case where a light emission ratio ofthe light emitting element of the pixel per frame (duty ratio) isfigured, and the light emitting element of the monitoring element aremade to emit light in accordance with the duty ratio. Namely, in thecase of the digital gradation system, the setting current of themonitoring element is preferably set at (current value during the lightemission×duty ratio).

In the case of an analog gradation method, the gradation is controlledby the amount of the current which flows into the light emitting elementas described above. Therefore, in the analog gradation system, a currentvalue over the average of the pixel with the maximum light emission ispreferably required.

That is to say, according to the invention, it is possible to obtain asetting voltage with an estimated deterioration of the light emittingelements of all the pixels by measuring the deterioration of themonitoring elements and setting the setting voltage of the monitoringelements in the saturation region.

Further, the deterioration ratio of the light emitting elements isdifferent depending on the materials: red (R), green (G) and blue (B).In this case, by taking the deterioration of the light emitting elementsof the monitoring elements equal to or more than the most deterioratedelement of each light emitting element, V₂ (V_(cathode)) can have asufficient value.

As above, according to the invention, it is possible to obtain a settingvoltage without a deterioration margin (voltage a). Accordingly, amargin of the setting voltage in accordance with a deterioration of alight emitting element is not required, thus heat generation and powerconsumption can be reduced. Particularly, by the reduction in powerconsumption of the driver transistor, the deterioration of the lightemitting element can be prevented.

Embodiment Mode 2

Hereinafter explained with reference to FIG. 4 is a different pixelstructure from that of Embodiment Mode 1. Note that in this embodimentmode, a driver transistor is a p-channel type, an anode potential of alight emitting element (V_(anode)) is referred to as V₁, and a cathodepotential of the light emitting element (V_(cathode)) is referred to asV₂.

An equivalent circuit diagram of a pixel structure is shown in FIG. 4.As well as Embodiment Mode 1, a monitoring element 401 has a drivertransistor 402 and a light emitting element 403 which is connected to asecond electrode of the driver transistor 402. A pixel 411 has a drivertransistor 412 and a light emitting element 413 which is connected to asecond electrode of the driver transistor 412. The light emittingelements 403 and 413 are connected to an output terminal of theoperational amplifier 420 and have a voltage of V₂. Meanwhile, firstelectrodes of the driver transistors 402 and 412 have potential V₁ whichis the same potential as the electrodes of the light emitting elements.

Unlike Embodiment Mode 1, a non-inverted input terminal (+side) of theoperational amplifier 420 is connected to a bias power source V_(b), andan inverted input terminal (−side) is connected to the interconnectionbetween the driver transistor of the monitoring element 402 and thelight emitting element 403.

Next, a method for setting a voltage of the driver transistor of themonitoring element (V_(ds)) is explained.

First, V₁ is set on the basis of a specification of a display device.Then, a gate voltage of the driver transistor 402 (V_(moni)) is set sothat a predetermined current (I_(ref)) can flow in the monitoringelement in a saturation region. Also, a gate voltage of the drivertransistor 412 (V_(pix)) is set at the same value as or higher thanV_(moni).

Next, V_(b) is determined so that the driver transistor 402 of themonitoring element can operate in a saturation region. Namely, V_(ds) ofthe driver transistor is determined. As described above, a current whichflows in the monitoring element 401 is determined by the operationalamplifier and V₂ (V_(cathode)) is determined so that the current whichflows in the monitoring element 401 can flow in the light emittingelement 403.

Namely, the values of V₁ and V_(b) are determined externally and V_(ds)of the driver transistor of the monitoring element is controlled.

When a display operation is started by setting as above, a resistancevalue rises due to the deterioration of the light emitting element.Then, with the current value flowing in the monitoring element lowered,V_(ds) of the driver transistor 402 also tries to lower itself. However,V_(ds) is held constant because the potential difference between theinput terminals of the operational amplifier is ideally 0. Then, I_(ds)becomes constant because V_(gs) and V_(ds) are constant, thus V₂ isautomatically selected by I_(ds).

The monitoring element and the pixel has V₂ (V_(cathode)) in common.That means V_(ds) of the driver transistor of the pixel is set as well.

In this manner, this embodiment mode is characterized by determiningV_(ds). Further, this embodiment mode in which V_(ds) is directlycontrolled provides a simpler method for setting a voltage as comparedto Embodiment Mode 1 in which I_(ds) is determined.

As described above, according to the invention, it is possible to obtaina setting voltage without a deterioration margin (voltage a) from thetime the light emitting element starts emitting light. Therefore, amargin of the setting voltage in accordance with a deterioration of thelight emitting element is not required, thus heat generation and powerconsumption can be reduced. Particularly, by the reduction in heatgeneration of the driver transistor, the deterioration of the lightemitting element can be prevented.

Embodiment Mode 3

Hereinafter explained is a different pixel structure from those ofEmbodiment Mode 1 and Embodiment Mode 2.

FIG. 10 shows a circuit diagram as described in Embodiment Mode 2,wherein a switching regulator 3000 is applied in stead of an operationalamplifier as a power source voltage controller. For a constitution wherean operational amplifier is applied, a power source circuit for theoperational amplifier is required. This embodiment mode makes itpossible to unite an operational amplifier and the power source circuitby using a switching regulator.

A pixel structure which has a switching regulator will be hereinafterdescribed. In FIG. 10, the switching regulator 3000 is comprised of anerror amplifier 3001, a PWN comparator 3002, a reference power sources3003 and 3010, an oscillation circuit 3004, a switching transistor 3008,an inductor 3009, a diode 3006, a smoothing capacitor 3005 and a battery3007. As well as Embodiment Mode 2, a monitoring element has a drivertransistor 3011 and a light emitting element 3012 which is connected toa first electrode of the driver transistor 3011. A pixel has a drivertransistor 3013 and a light emitting element 3014 which is connected toa first electrode of the driver transistor 3013. Gate electrodes of thedriver transistors 3011 and 3013 are connected to a power source 3015and second electrodes of the transistors 3011 and 3013 are connected toa power source 3016.

Next, an operation of the switching regulator will be explained. At thestart of operating, a potential of the smoothing capacitor 3005 which isan output of the switching regulator is 0. The potential of thesmoothing capacitor is inputted to an inverted input terminal of theerror amplifier 3001, and a potential of the light emitting element isinputted to a non-inverted input terminal. A current of the transistor3011 flows in the light emitting element 3012 and a voltage is generatedin the light emitting element. When the voltage is higher than that ofthe reference power source 3010, the error amplifier 3001 operates so asto lower the output. Then, the PWN comparator 3002 operates so as tolower the voltage of the inductor 3009 by changing the duty of theoscillation. Therefore, a potential of the smoothing capacitor 3005 islowered and an anode potential of the light emitting element 3012 isalso lowered to become approximately the same potential as the powersource 3010. Meanwhile, when an anode potential of the light emittingelement 3012 is lower than that of the reference power source 3010, theopposite operation is taken, and the anode potential rises to the samepotential as the reference power source 3010.

In this manner, the same effect as that of an operational amplifier canbe obtained by using the switching regulator 3000. Also, a power sourcecan be reduced.

Embodiment Mode 4

Hereinafter explained with reference to FIGS. 5 and 6 is the pixelportion having a monitoring element. Note that a thin film transistor(hereinafter referred to as TFT) formed over an insulating surface isemployed as a transistor of an active element in this embodiment mode.

Shown in FIG. 5 is an equivalent circuit diagram of a pixel portion 500having a first dummy pixel 501, a monitoring element 502, a second dummypixel 503 and a pixel 504 in this order. The first and second dummypixels are provided so that the whole pixel portion is under the equalcondition including the pixels at the edge as well as the pixels aroundthem.

The dummy pixel, the monitoring element and the pixel have similarstructures, having a first TFT (selector TFT) 511, a second TFT (erasureTFT) 512, a third TFT (driver TFT) 513, a capacitor element 514, and alight emitting element 515 at the crossed part of a signal line 521 anda first scanning line 522. Note that the selector TFT and the erasureTFT are formed by using n-channel type TFTs, and the driver TFT isformed by using a p-channel type TFT in this embodiment mode. Also, asecond scanning line 523 which is connected to a gate electrode of theerasure TFT, and a current supply line 524 which is connected to a firstelectrode of the erasure TFT and a first electrode of the driver TFT areprovided.

The dummy pixel, the monitoring element and the pixel, however, vary inconnections of each structure. First and second dummy pixels are notconnected to the first electrode of the selector TFT 511 and the signalline 521. Secondly, the first electrode of the driver TFT 513 is notconnected to the first electrode of the light emitting element 515.These dummy pixels are provided in order to operate the whole pixelsunder the same condition including the pixels at the edge as well as thepixels around them. Therefore, dummy pixels neither need to emit light,write data from the signal line to pixels, nor make the light emittingelement emit light. In the invention, however, the dummy pixel may emitlight. The signal line 521 of the dummy pixel and the current supplyline 524 are connected to each other to have the same potentials.

In the monitoring element, the first electrode of the selector TFT 511is not connected to the signal line 521. The signal line 521, however,is connected to the first electrode of the light emitting element 515.This is intended to make the monitoring element emit light constantly sothat the deterioration thereof proceeds fast. Therefore, voltage appliedfrom the signal line as information for brightness does not have to gothrough the selector TFT 511 to be inputted to the light emittingelement. The signal line 521 and the current supply line 524 of themonitor element are connected to an operational amplifier respectively.

In the pixel, the first electrode of the selector TFT 511 is connectedto the signal line 521, and a first electrode of the driver TFT 513 isconnected to the first electrode of the light emitting element 515. Itis intended that in the pixel, the light emitting element 515 emitslight through the driver TFT 513 based on the signal voltage from thesignal line. Furthermore, the signal line 521 and the current supplyline 524 of the pixel are connected to the driver circuit and an FPCrespectively.

In FIG. 6, a top plan view of a part of the pixel portion shown in FIG.5 is shown. The first dummy pixel 501, the monitoring element 502, thesecond dummy pixel 503, and the pixel 504 of the first line are shown.These dummy pixels, monitoring elements and pixels have the selector TFT511, the erasure TFT 512, the driver TFT 513, and the light emittingelement 515 (only the first electrode thereof is shown) at the crossedparts of the signal line 521, the current supply line 524, the firstscanning line 522, and the second scanning line 523. A capacitor element514 (configured with a gate metal and a semiconductor film of the TFT513) is provided as needed. Note that, another capacitor element isadded when the gate capacitor of the driver TFT is too small for theleakage current of the TFT.

As described above with reference to FIG. 5, these dummy pixels, themonitoring element, and the pixel have the identical structures,however, the presence of contacts differs. That is, what differs in thedummy pixels, the monitoring element, and the pixel is whether theconnection between the selector TFT 511 and the signal line 521 and theconnection between the driver TFT 513 and the light emitting element 515exist or not.

In the first and second dummy pixels, there are no contacts in thecontact portion of the selector TFT 511 and the signal line 521, and inthe contact portion of the driver TFT 513 and the first electrode of thelight emitting element 515. The monitoring element, however, is providedwith a contact 601 with the signal line 521 in the contact portion ofthe driver TFT 513 and the light emitting element 515, although there isno contact in the contact portion of the selector TFT 511 and the signalline 521. In the pixel, there is a contact 602 in the contact portion ofthe selector TFT 511 and the signal line 521, and a contact 603 in thecontact portion of the driver TFT 513 and the light emitting element515.

Furthermore, a leading wiring is provided so that the signal line andthe current supply line of the monitoring element are connected to theoperational amplifier. Moreover, the signal line and the current supplyline of the pixel are connected to an FPC terminal 506 or a drivercircuit respectively. The signal line and the current supply line of thedummy pixel are connected to each other and have the same potentials.

The monitoring element is not necessarily required in a whole line, buthas only to be provided one. It depends on the performance of theoperational amplifier to which the monitoring element is connected. Themonitoring elements may be provided in plural, and also can be disposedsymmetrically to the pixel portion. The monitoring element may bedisposed in any forms.

A line of monitoring elements are connected to each other in parallelthrough the current supply line, and a plurality of monitoring elementscan be seen as one big monitoring element.

In this manner, the monitoring element of the invention can be formed bychanging the layout design of the element, without changing the processof the pixel. Also, the setting voltage of the pixel can be at the bestvoltage in the saturation region at all times by utilizing themonitoring element formed thereby. Therefore, heat generation and powerconsumption can be reduced, resulting in the longer life of the lightemitting element.

Embodiment Mode 5

The pixel portion shown in the above embodiment mode is provided lightemitting elements and sealed not to be exposed to the air, thuscompleting a panel. ICs including an operational amplifier, acontroller, and a power source circuit and the like are mounted on thepanel, thus completing a display module. The specific structure of thedisplay module is explained here.

A pattern diagram provided with a line of monitoring elements 751 nearthe signal line driver circuit 705 is shown in FIG. 11A. The monitoringelement shown in FIG. 11A comprises a signal line driving circuit 4001,a scanning line driving circuit 4002, a plurality of dummy pixels 4003,a plurality of pixels 4004 and a monitoring element 4005. The inventionhas monochrome light emitting elements when the monitoring elements areprovided in one line as shown in FIG. 11A. Therefore, it is desirable toapply it to a display device which expresses RGB with a color convertinglayer.

Furthermore, the monitoring elements may be provided in a plurality oflines or at a plurality of portions as shown in FIG. 11B. The monitoringelement shown in FIG. 11A comprises a signal line driving circuit 4001,a scanning line driving circuit 4002, a plurality of dummy pixels 4003,a plurality of pixels 4004, a 1st monitoring element (R) 4006, a 1stmonitoring element (G) 4007 and a 1st monitoring element (B) 4008. Whenproviding the monitoring elements in a large panel particularly, it isdesirable that the monitoring elements be provided in a plurality oflines although it depends on the performance of the operationalamplifier. At the same time, first to third monitoring elements hadbetter be provided in consideration of the difference of deteriorationsbetween the materials for each color (RGB) as shown in FIG. 11B. Theplace to dispose each monitoring element is not exclusively applied toFIG. 11B. It may be disposed at any peripheral region of the pixel,including outside thereof.

An outline view of the display module of the structure of FIG. 11A isshown in FIG. 7A. The display module is mounting an operationalamplifier 750, a controller 701 and a power source circuit 702. A pixelportion 703 in which a light emitting element is formed in each pixel, amonitoring element 751 and a dummy pixel 752 are mounted, a scanningline driver circuit 704 for selecting pixels in the pixel portion 703,and a signal line driver circuit 705 for supplying a video signal to theselected pixels are formed in the panel 700. The monitoring element 751is disposed near the signal line driver circuit 705 as one side of thepixel portion 703 and connected to the operational amplifier 750. Inaddition, the dummy pixel 752 is provided around (periphery of) thepixel portion 703 in order to put the pixels at the edge (the mostoutside pixels. In case of m×n pixels, and a pixel in the first row andthe first line, a pixel in the m-th row and the n-th line) under thesame condition as the peripheral pixels.

Similarly, the dummy pixel 752 may be disposed at the necessaryperiphery only although it is disposed around the pixel portion 703 inthe figure. Also, the place and number of the signal line driver circuitand the scanning line driver circuit to dispose are not limited to FIG.7A.

Further, the operational amplifier 750, the controller 701 and the powersource circuit 702 are formed over a printed circuit board 706. Eachtype of signal and a power source voltage outputted from the controller701 or the power source circuit 702 are supplied through an FPC 707 tothe pixel portion 703, the scanning line driver circuit 704, and thesignal line driver circuit 705 of the panel 700.

The power source voltage and each type of signal are supplied to theprinted circuit board 706 through an interface (I/F) portion 708 onwhich a plurality of input terminals are disposed.

Note that although the printed circuit board 706 is mounted via an FPC707 on the panel 700 in this embodiment mode, the structure is notnecessarily limited to this. The controller 701 and the power sourcecircuit 702 may also be mounted directly on the panel 700 by using a COG(Chip on Glass) method.

Further, noise may ride on the power source voltage and the signals, andthe signal rise time may become slowed, due to capacitance that areformed between leading wirings, resistance of wirings themselves, andthe like of the printed circuit board 706. Various types of elements,such as capacitors and buffers, may be formed over the printed circuitboard 706 so as to prevent noise from riding on the power source voltageor the signals, and slowness in the signal rise time.

A block diagram of the structure of the printed circuit board 706 isshown in FIG. 7B. Each type of signal and the power source voltagesupplied by the interface 708 are supplied to the controller 701 and thepower source circuit 702.

The controller 701 has an A/D converter 709, a phase locked loop (PLL)710, a control signal generator portion 711, and SRAMs (Static RandomAccess Memories) 712 and 713. Note that, although SRAMs are used here,it is also possible to use SDRAMs or DRAMs (Dynamic Random AccessMemories) as substitutes for the SRAMs, provided that the DRAMs arecapable of writing and reading data at a high speed.

A video signal supplied through the interface 708 is subjected to aparallel-serial conversion in the A/D converter 709, and inputted to thecontrol signal generator portion 711 as a video signal corresponding tothe colors R, G and B (R video signal 714, G video signal 715 and Bvideo signal 716). Further, an Hsync signal 717, a Vsync signal 718, aclock signal CLK (CLK1 719 and CLK2 720), and alternating voltage (ACCont 721) are generated in the A/D converter 709 based on each of thesignals supplied through the interface 708, and then inputted to thecontrol signal generator portion 711.

The phase locked loop 710 functions to align the phase of the operatingfrequency of the control signal generator portion 711 with the frequencyof each of the signals supplied through the interface 708. The operatingfrequency of the control signal generator portion 711 is not necessarilythe same as the frequency of each of the signals supplied through theinterface 708. The operating frequency of the control signal generatorportion 711 is regulated in the phase locked loop 710 so that thefrequencies become synchronized.

The video signal inputted to the control signal generator portion 711 istemporarily written into the SRAMs 712 and 713, and stored. A videosignal corresponding to all pixels is read out one bit at a time fromthe video signals of all of the bits that are stored in the SRAM 712,and then supplied to the signal line driver circuit 705 of the panel700.

The control signal generator portion 711 supplies information, whichrelates to light emission periods by the light emitting elements foreach bit, to the scanning line driver circuit 704 of the panel 700.

The power source circuit 702 supplies a predetermined power sourcevoltage to the signal line driver circuit 705, the scanning line drivercircuit 704, and the pixel portion 703 of the panel 700.

The display module formed like this can set the best setting voltage inthe saturation region all the time by utilizing the monitoring element.Therefore, heat generation and power consumption can be reduced,resulting in the longer life of the light emitting element.

Embodiment Mode 6

Given as examples of electronic apparatuses that employ display devicesmanufactured in accordance with the invention are video cameras, digitalcameras, goggle type displays (head mounted displays), navigationsystems, audio playback devices (car audios, audio components, etc.),notebook type personal computers, game machines, portable informationterminals (mobile computers, mobile telephones, mobile type gamemachines, and electronic books, etc.), image reproduction devicesequipped with a recording medium (specifically, devices equipped with adisplay device capable of reproducing the recording medium such as aDigital Versatile Disk (DVD), etc. and displaying the image thereof),and the like. Examples of these electronic apparatuses are shown in FIG.9.

FIG. 9A is a display device, which is composed of a frame 2001, asupport base 2002, a display portion 2003, a speaker portion 2004, avideo input terminal 2005, and the like. The pixel portion having amonitoring element of the invention is used for the display portion 2003to manufacture the display device. Note that in the case ofmanufacturing a large-sized display device, the monitoring elements areprovided in a plurality of rows or more preferably at every RGBs. Whenthe invention is applied to such a display device, particularly to alarge-sized display device, the low power consumption is achieved, thusthe problems of the heat generation and the deterioration of the lightemitting elements can be solved. Note that the term display deviceincludes all display devices for displaying information, such as thosefor personal computers, those for receiving TV broadcasting, and thosefor advertising.

FIG. 9B is a digital still camera, which is composed of a main body2101, a display portion 2102, an image-receiving portion 2103, operationkeys 2104, an external connection port 2105, a shutter 2106, and thelike. The pixel portion having a monitoring element of the invention isused for the display portion 2102 to manufacture the digital stillcamera.

FIG. 9C is a notebook type personal computer, which is composed of amain body 2201, a frame 2202, a display portion 2203, a keyboard 2204,an external connection port 2205, a pointing mouse 2206, and the like.The pixel portion having a monitoring element of the invention is usedfor the display portion 2203 to manufacture the notebook type personalcomputer.

FIG. 9D is a mobile computer, which is composed of a main body 2301, adisplay portion 2302, a switch 2303, operation keys 2304, an infraredport 2305, and the like. The pixel portion having a monitoring elementof the invention is used for the display portion 2302 to manufacture themobile computer.

Mobile apparatuses such as a notebook type personal computer and amobile computer which has a pixel portion having a monitoring element ofthe invention have advantages in that power consumption is reduced andthe storage period of a battery is prolonged.

FIG. 9E is a portable image reproduction device provided with arecording medium (specifically, a DVD playback device), which iscomposed of a main body 2401, a frame 2402, a display portion A 2403, adisplay portion B 2404, a recording medium (such as a DVD) read-inportion 2405, operation keys 2406, a speaker portion 2407, and the like.The display portion A 2403 mainly displays image information, and thedisplay portion B 2404 mainly displays character information, and thepixel portion having a monitoring element of the invention is used inthe display portion A 2403 and in the display portion B 2404 tomanufacture the portable image reproduction device. Note that imagereproduction devices provided with a recording medium include gamemachines for domestic use and the like.

FIG. 9F is a goggle type display (head mounted display) which iscomposed of a main body 2501, a display portion 2502, an arm 2503. Thepixel portion having a monitoring element of the invention is used inthe display portion 2502 to manufacture the goggle type display.

FIG. 9G is a video camera, which is composed of a main body 2601, adisplay portion 2602, a frame 2603, an external connection port 2604, aremote control receiving portion 2605, an image receiving portion 2606,a battery 2607, an audio input portion 2608, operation keys 2609, aneyepiece portion 2610, and the like. The pixel portion having amonitoring element of the invention is used for the display portion 2602to manufacture the video camera.

FIG. 9H is a mobile telephone, which is composed of a main body 2701, aframe 2702, a display portion 2703, an audio input portion 2704, anaudio output portion 2705, operation keys 2706, an external connectionport 2707, an antenna 2708, and the like. The pixel portion having amonitoring element of the invention is used for the display portion 2703to manufacture the mobile telephone. Note that by displaying whitecharacters on a black background, the display portion 2703 can suppresspower consumption of the mobile telephone.

As described above, the application scope of the invention is so widethat it can be used in electronic apparatuses of various fields,particularly to a flat panel display.

EMBODIMENT Embodiment 1

Hereinafter explained is an experimental result with respect to a changeof a cathode voltage (cathode potential) corresponding to a secularchange and a current which is supplied to a light emitting element.Incidentally, this embodiment mode applies an experimental circuit towhich a circuit as shown in FIG. 10 is applied.

FIG. 12 shows a circuit diagram of this embodiment to which a circuitdiagram of FIG. 10 is applied. For a power source circuit, a μPC1100(produced by NEC Corporation) is used. A resistance R1 is set so that avoltage of a VMO terminal can be approximately 1.05 to 1.45V. Aresistance R2 is set so that a driver TFT of a monitoring element canoperate in a saturation region. A resistance R3 is set so that a voltageof a DTC terminal can be approximately 1.87V. A power source voltage Vccis set at a voltage of 7V. A CATHODE terminal is connected to a cathodeof a light emitting element of a pixel and a cathode of a light emittingelement of the monitoring element. A MONITOR terminal is connected to ananode of the light emitting element of the monitoring element.Meanwhile, second electrodes of driver TFTs of the pixel and themonitoring element and which are not connected to the light emittingelements are fixed at 5V.

Shown in FIGS. 13 and 14 are the results of experiment conducted byusing the circuit as shown in FIG. 12. FIG. 13 shows a change of acathode potential (V_(cathode)) with the passage of time (hour). It canbe confirmed that an absolute value of the cathode potential(V_(cathode)) rises as time passes. Meanwhile, FIG. 14 shows a currentvalue (I) which is supplied to a light emitting element with the passageof time (hour). It can be confirmed that a constant current value issupplied to the light emitting element. Note that the current valuesupplied to the light emitting element is equal to the currentconsumption.

The longer the light emitting element emits light, the faster the lightemitting element deteriorates and the higher the absolute value of therequired cathode voltage becomes. However, the current value supplied tothe light emitting element does not change. Accordingly, the circuit ofthe invention can control the cathode voltage normally in order to keepthe current value supplied to the light emitting element constant.

By using the pixel structure provided with the circuit of the invention,it is possible to operate a diver TFT in a saturation region without adeterioration margin of the light emitting element, or with a smallermargin than the conventional one. Therefore, heat generation and powerconsumption can be reduced.

The invention makes it possible to set a voltage in a saturation regionwithout a deterioration margin of a light emitting element, or with asmaller margin than the conventional one from the time the lightemitting element starts emitting light. Therefore, the margin of thesetting voltage in accordance with the deterioration of the lightemitting element is not required, leading to the reduced heat generationand power consumption. Further, the deterioration of the light emittingelement can be prevented particularly because heat generation of thedriver transistor is reduced.

1. A display device comprising: a first transistor; a second transistor;a third transistor electrically connected to the second transistor; afirst light emitting element including a first electrode; and a secondlight emitting element including a second electrode, wherein the firstelectrode overlaps with at least a part of the first transistor, whereinthe second electrode overlaps with at least a part of the secondtransistor, wherein the first transistor is electrically connected tothe first electrode, wherein the second transistor is not electricallyconnected to the second electrode, wherein a signal line overlaps with apart of the third transistor, and wherein the third transistor is notelectrically connected to the signal line.
 2. A display device accordingto claim 1, wherein the first transistor and the second transistor arethin film transistors.
 3. A display device comprising: a firsttransistor; a second transistor; a third transistor electricallyconnected to the second transistor; an insulating film over the firsttransistor and the second transistor; a first light emitting elementincluding a first electrode on the insulating film; and a second lightemitting element including a second electrode on the insulating film,wherein the first electrode overlaps with at least a part of the firsttransistor, wherein the second electrode overlaps with at least a partof the second transistor, wherein a contact hole is provided in theinsulating film, and the first transistor is electrically connected tothe first electrode through the contact hole, wherein the secondtransistor is not electrically connected to the second electrode,wherein a signal line overlaps with a part of the third transistor, andwherein the third transistor is not electrically connected to the signalline.
 4. A display device according to claim 3, wherein the firsttransistor and the second transistor are thin film transistors.
 5. Alight emitting device comprising: a first transistor and a first lightemitting element having a first electrode connected to a first electrodeof the first transistor; a monitoring element having a second transistorand a second light emitting element having a first electrode connectedto a first electrode of the second transistor; and a power sourcevoltage controller, wherein an input terminal of the power sourcevoltage controller is connected to second electrodes of the first andsecond transistors respectively; and an output terminal of the powersource voltage controller is connected to second electrodes of the firstand second light emitting elements.
 6. A light emitting device accordingto claim 5, wherein the power source voltage controller is anoperational amplifier.
 7. A light emitting device according to claim 5,wherein the power source voltage controller is a switching regulator.